It has been known heretofore that a cutting resistance exerted on a workpiece is remarkably reduced if ultrasonic vibration is applied in a cutting direction to a tool held on a tool rest in a lathe or the like, whereby any abnormal vibration such as irregular shaking is eliminated to eventually enhance the cutting precision relative to the deviation from circularity of the workpiece and the surface roughness thereof with further advantages of prolonging the service life of the tool as well as facilitating the machining of a hard-to-cut material. And the desired effect is attained when the condition V&lt;2.pi.fa is satisfied, in which v is the velocity of cutting the workpiece, f is the frequency of vibrating the tool, and a is the amplitude of such vibration.
Referring now to FIG. 8, a description will be given on an exemplary cutting method carried out by a flexural vibratory tool holder which utilizes a typical vibrator of vertical oscillation system. First a flexural vibratory tool holder 5 is supported on a tool rest 1 by means of keep plates 2, clamp bolts 3 and clamp members 4. In this example, the clamp members 4 are positioned at nodes of the tool holder 5. Then a cutting tool 7 is anchored to the fore end of the tool holder 5 so as to be set opposite to a workpice 6, and both a vertical vibrator 8 and a vibration amplifier horn 9 are joined together on a vibrational loop of the tool holder 5 represented by one-dot chain lines. When the vertical vibrator 8 is driven in such arrangement, the tool holder 5 is vibrated as shown by one-dot chain lines, so that the cutting tool 7 held at the fore end thereof is also vibrated at an ultrasonic frequency in the cutting direction, hence exhibiting the required cutting effect with vibration as mentioned previously.
The apparatus of such a structure is advantageous in the point that the vertical vibrator 8 can be disposed at a positioned spaced apart from the cutting tool and is thereby attachable to an ordinary lathe with facility. However, there also exists a disadvantage that the vertical vibrator 8 is positionally impedimental to the operation and great difficulty is unavoidable in mounting the flexural vibratory tool holder 5 on the tool rest 1. Unless each of the clamp members 4 is set exactly at a proper node, there arises a problem of energy loss and the edge of the cutting tool 7 comes to vibrate in a harmful direction while firm setting thereof is requisite, hence necessitating complicated labor and great care for replacement of the tool holder 5.
Furthermore, at the point of contact between the cutting tool 7 and the workpiece 6 in the apparatus of FIG. 8, a diametrical back component force Pt is generated toward the cutting tool 7 in addition to the main component force Pc tangential to the workpiece as shown in FIG. 9, so that the resultant force P is exerted on the cutting tool to eventually produce harmful abnormal vibration, which may therefore bring about failure in achieving sufficient vibrational cutting effect. In an attempt to solve the problems mentioned above, an improvement has been proposed as disclosed in Japanese Patent Publication No. 50-20289, wherein the flexural vibratory tool holder 5 is installed with an inclination relative to the cutting direction in such a manner that a directional coincidence is attained between the vibration of the cutting tool 7 and the resultant force P. However, for enabling such inclined installation, it becomes necessary to prepare additional provision of a particular index head, an inclined table and so forth, thereby rendering the apparatus structure complicated.